An antilock brake system (ABS) (or antiskid system) is a system for shortening vehicle braking distance as much as possible by controlling a brake so as to obtain a slip factor that maximizes braking force. Since a slip factor for maximizing braking force differs depending upon the condition of the road surface, it is necessary that the condition of the road surface be discriminated so as to carry out optimum ABS control suited to the conditions of the road surface. In addition, it is necessary to identify very bumpy gravel roads in order to absorb unpleasant vibration and maintain ideal riding comfort by controlling the suspension in conformity with the roughness of the road surface.
One technique for optically discriminating road surface conditions without contact is disclosed in "Road-Surface Condition Sensor Utilizing Polarizing Characteristic of Road Surface Reflection", by Toshio Takehana, Hikari Gijutsu Contact, Vol. 27, No. 3 (1989), pp. 158-164. According to this technique, a light-emitting element and a light-receiving element are arranged in such a manner that angle of incidence and angle of reflection take on the value of the Brewster angle (53.degree.). This utilizes the fact that since a wet road surface approximates a mirror surface, the degree of polarization approaches unity, whereas in the case of a dry road surface, the degree of polarization approaches zero.
However, this technique is such that an asphalt road surface is identified as being in either a wet condition or a dry condition; gravel roads and snow-covered roads cannot be discriminated. Further, since the angles at which the light-emitting element and light-receiving element are arranged are determined by the Brewster angle, precise positioning is required and the light-transmitting and light-receiving elements must be provided quite far apart.
Further, road-surface condition estimation suited for application to antiskid control is available ("Road-Surface Estimation Using Fuzzy Logic", by Katsuhiro Oba et. al., Jidosha Gijutsukai, Corporate Juridical Person, Gakujutsu Koenkai Zensatsushu 881, May, 1988, 881028). However, since this technique is suitable for application to an antiskid system, it lacks universality.